1. Field of the Invention
This invention is related to a microstrip antenna having a plate-shaped radio frequency radiation conductor.
2. Description of the Related Art
The microstrip antennas used in conventional portable radio equipment are equipped with a basic structure as indicated, for example, in Japanese Patent Application Laid-Open No. 3-166802. In brief, this is a structure in which a ground conductor plate such as a conductor plate of copper foil, etc., a dielectric substrate such as a resin substrate, and a radiation conductor plate of the same material as the ground conductor plate but with a smaller area are laminated into layers. The radiation conductor plate is square or rectangular and is positioned roughly on the central part of the dielectric substrate. The radiation conductor plate is connected to the ground conductor plate by multiple through-holes formed on its periphery. A feeding point is formed somewhat off from the central part of the radiation conductor plate toward the side of the through holes.
This antenna is characterized by having a directional pattern that radiates direct waves of roughly uniform strength in all directions of the radiation surface (plate surface) of a radiation conductor plate 1. It has antenna gain in the direction to the rear of the radiation conductor plate as well.
With a radio device that transmits and receives radio signals by an antenna as described above, it is generally necessary to minimize the reception of noise generated from sources of noise in the vicinity. Conventionally, in order to avoid the influence of noise, noise shields have been employed or the antenna has been placed in a position where it is less likely to be influenced by noise. However, there is a problem with noise source shields, in that there is a large burden imposed by the attendant increase in cost. There is also a problem with placing the antenna in a position where it is less likely to be influenced by noise, in that this is limited by the size of the antenna.
It is also possible to lessen the noise reception of a radio device by reducing antenna gain in the direction of the noise source, by controlling the antenna directivity by using a microstrip antenna with an array structure, like that in Japanese Patent Application Laid-Open No. 4-160801. Moreover, directivity can be sharpened to some extent by making the area of the ground conductor plate considerably larger than the radiation conductor plate.
However, devices with these structures require a large area, making it difficult to apply them in radio devices for portable use, for which small size and light weight are demanded.